Cable safety systems and cable barriers have been installed along edges of roadways and highways for many years. Cable safety systems and cable barriers have also been installed along medians between roadways and/or highways. Cable safety systems generally include one or more horizontal cables attached to support posts. For some applications cable safety systems and cable barriers may reduce damage to an impacting vehicle and/or injury to occupants of the impacting vehicle as compared with other types of highway safety systems and highway barriers.
Cable safety systems are often designed and installed with at least one cable mounted horizontally on a plurality of generally vertical support posts. Many cable safety systems include three cables spaced vertically from each other on each support post. The number of cables may vary depending on factors such as the type of vehicles using the associated roadway and the hazard which requires installation of the cable safety system. The length of a cable safety system is generally determined based on the adjacent roadside hazard. Each cable is typically installed at a selected height relative to the ground and with selected vertical spacing between adjacent cables.
One recognized limitation of many cable safety systems is excessive deflection of associated cables during a vehicle impact. Deflection associated with a cable safety system may be larger than deflection of a convention W-beam guardrail when subjected to the same type of vehicle impact. Such deflection frequently determines maximum allowed spacing between adjacent posts for satisfactory performance of the cable safety system. Large deflection during a vehicle impact also increases the risk of the vehicle running over the cables and being exposed to the hazard which required installation of the cable safety system. Calculating performance of many cable safety systems is often difficult due to unpredictable interactions between associated posts and cables during a vehicle impact. Depending upon car type, speed and angle of impact, cables may release as far as ten (10) or most posts spaced ahead of the impact location. Cable release from post often causes much larger deflections than expected or calculated.
From full scale crash testing and from real life experience, it has been determined that keeping the length of unsupported cables as short as possible will generally reduce deflection. The longer the distance between adjacent posts supporting associated cables, the larger the deflection will generally be during a vehicle impact. An increased number of posts (shorter post spacing) will generally decrease deflection. However, shorter spacing between posts affects total cost of a cable safety system, not only material, but also installation time and cost.
During the past several years, cable safety systems have been used as an alternative to traditional W-beam guardrail systems. These cable safety systems address some of the weaknesses of prior cable safety systems by using pre-stressed cables and/or reducing spacing between adjacent posts to reduce deflection to an acceptable level. A consultant report “Dynamic Analysis of Cable Guardrail” issued in April 1994 by an ES-Consult in Denmark, established a model for various parameters which affect performance and design considerations for acceptable deflection of cable safety systems.
Cable safety systems are often more aesthetically appealing and minimize potential sight distance problems as compared with W-beam and thrie beam guardrail systems. Cable safety systems generally minimize snow accumulation on adjacent highways and roadways.